The development of new metal alloys (a field called metallurgy) is limited by the foundry processes used for alloying. New metal alloys must typically be melted at high temperatures, allowed to mix to promote homogeneity, and are then cast into billets of a single composition. Due to the nature of conventional homogenous alloying, it is extremely difficult to fabricate a piece of metal that has a gradient of compositions from one metal to another. Yet such gradient properties would be extremely useful, based on the wide assortments of “post processing” methods often employed to locally change the properties of a metal alloy. For example, the contact surfaces on the teeth of metal gears need to be hard to avoid abrasive wearing while the bulk of the gear needs to be tough to support the loads without failing. To do this, the teeth in some cases are infused with carbon in a process called “case hardening” to change the properties only in the desired area. In another example, shot peening the surface of a brittle metal may be used to improve the fatigue properties locally near the surface by creating a gradient of mechanical properties from the interior of the part to the exterior. While these post-processing techniques are able to modify some properties of some materials, there are substantial limitations both as to the types of properties that can be modified, and to the materials that can undergo such modification.
Recently, Additive Manufacturing (AM) techniques have been developed that may shift the paradigm of traditional metal production. These AM processes are typically designed as 3D printing techniques for fabricating metal alloys into net-shapes, as is common with plastics. Traditionally, a complex part is fabricated either by molten casting or molding, or from a solid piece of metal by machining a large billet whose overall dimension is larger than the outer-most piece of the finished hardware. AM techniques build the part up by depositing material layer-by-layer using computer aided processing. Only the material needed in the final part is deposited, thus eliminating the need for complex machining.
One AM technology that has promise is Laser Engineered Net Shaping (LENS). LENS is an additive manufacturing process for fabricating metal parts from a Computer Aided Design (CAD) solid model. It is very similar to other rapid prototyping techniques in that it fabricates a solid part by layers at a time, however, the LENS technology is unique in that it is able to produce fully dense parts with material properties similar to its wrought counterparts. The novelty of LENS lies in its multi-nozzle capability and precision deposition.
The LENS process fabricates a part by a laser head emitting a beam onto a substrate mounted on a worktable, simultaneously injecting metal powder into the molten pool from its powder delivery nozzles. These nozzles are connected to a hopper (a maximum of four hoppers using currently available equipment), delivering the powdered metal to the work zone. Either substrate or laser head is moved in the X-Y direction to deposit a thin layer of metal, creating the geometry of a part. After a layer is deposited, the laser head and powder nozzles move incrementally in the positive Z-direction, creating a 3-dimensional part. (See, Griffith, M. L. et al., Sandia Report—SAND2000-1000C: 18 May 2000, the disclosure of which is incorporated herein by reference.) Because multiple nozzles and multiple powder feedstocks may be used, LENS has the ability to mix powder streams of different materials, thereby producing components with precise composition control. It has been theorized that this capability to alter composition may be useful for minimizing the stress effects of mismatched coefficients of thermal expansion between dissimilar materials during thermal cycle and optimizing the mechanical properties critical to component performance.
A number of groups have identified the LENS technique's potential to allow for changing geometric properties (surface finish, part size, etc.) and material properties (coefficient of thermal expansion, tensile strength, etc.) within an article of manufacture to create truly multi-functional graded materials and parts. (See, Griffith, M. L. et al., cited above.) For example, LENS and other direct laser metal deposition techniques have been used to fabricate functionally graded materials for metallurgical research. Many graded materials have been deposited, such as iron-to-manganese and iron-to-nickel to study LENS process variables (laser power, powder feeder speeds, layer thickness, etc.) by analyzing melt-pool size and porosity; and Titanium-to-niobium graded compositions were 3D printed to present the capability of controlling properties of fabricated parts. (Atwood, C. J.; et al., Sandia Report—SAND2007-7832: November 2007; and Lewis, G. K., and Schlienger, E., Materials and Design, 21, 417, 2000, the disclosures of which are incorporated herein by reference.) However, these studies invariably stress the singular and non-generalizable nature of these results.
Accordingly, a need exists for a generalized technique that would allow a manufacturer to identify a set of different desired physical/chemical/electrical/magnetic/properties within a part, identify the terminal materials that would be needed to generate those properties, and then formulate a method of determining the necessary material gradients to allow for the changes in physical properties.